The thyristor family of semiconductor devices consists of several very useful devices, including silicon controlled rectifiers (SCRs), DIACS, and TRIACS. Thyristors are characterized by having two states, an on-state and an off-state, making them useful for electric power regulation, switching, and phase control.
An SCR is a unidirectional PNPN semiconductor device having an anode, a cathode, and a gate. In the off-state, an SCR is a high-resistance, low-current circuit element at voltages up to its breakover voltage. A momentary positive pulse applied to the gate of an SCR will bias the device so that it switches to the on-state and becomes a low-resistance, high-current element. Once an SCR is in the on-state, it will remain in the on-state until the principal anode to cathode current drops below the holding current of the device.
A TRIAC is a bidirectional thyristor which functions to control power in an AC electric circuit. The operation of a TRIAC can be related to two SCRs connected in inverse parallel. A TRIAC has a single gate, and the device can be triggered to the on-state by a gate pulse of either polarity. A TRIAC operates in a circuit in both directions essentially the same as an SCR behaves in its forward direction.
A DIAC is essentially an open base NPN transistor. A DIAC functions as a bidirectional circuit element which exhibits a high impedance state at voltages up to its voltage breakover point. Above the voltage breakover point a DIAC enters its on-state region, where current flows through the device as it exhibits decreasing resisitivity. Because of these characteristics, a DIAC can be used in a resistor-capacitor AC circuit to produce a bidirectional pulsing oscillator. In addition, a DIAC is an economical bidirectional device for triggering TRIACs in phase control circuits.
In many electrical applications it is desirable to have a power control circuit having a low voltage, low current triggering response with a relatively high holding current requirement. The high holding current can be provided by a TRIAC, but a standard DIAC does not provide the desired triggering characteristics for a TRIAC in such an application. This is because a standard DIAC has a higher voltage requirement and does not switch on completely at the breakover point. To achieve low voltage triggering capability for TRIACs, it has been necessary to construct relatively complicated triggering circuits comprising a plurality of transistors, resistors, and diodes in a bridge network. Such circuits have been simplified only to a limited extent by placing two integrated circuits on a chip to produce a silicon bilateral switch (SBS) such as the General Electric type 2N4991.
Because of the complexity of the prior art TRIAC triggering circuits, it is desirable to provide a simplified, economical, bidirectional, low voltage trigger device which switches on fully when the trigger voltage is reached.
A specific application which requires a low voltage trigger switch for a TRIAC is the maintenance termination unit (MTU) of a telephone line. The MTU is a diagnostic circuit placed at the subscriber's terminal of a telephone line and used for analyzing faults and maintaining the line. An MTU provided by the Western Electric Company utilizes a complicated trigger circuit to provide the specific low voltage switching capability necessary for the desired MTU response characteristics. An additional line testing device is described in U.S. Pat. No. 4,169,220 issued to Gary Fields on Sept. 25, 1979 and entitled "Telephone Instrument Connection block with Remotely Actuated Line Test". Because of the very large number of MTUs required to update telephone systems around the country, it is highly desirable to provide a simple, economical trigger switch to replace the complicated circuitry now used as a bidirectional, low voltage switch for the TRIAC in the MTU.